17-05-2011, 12:58 PM
(This post was last modified: 22-05-2011, 03:50 AM by Jeffrey Orling.)
Lewis raises another straw man argument.
I do not present a theory or an hypothesis but present the engineering principles in use every day to design structures.
The strength of materials has been tested and is presented in design load tables. Take out your ASTM steel design manual or even Wood Structural Design data for example and you will find the design loads for various types of steel or wood, sections, connections and so forth. The assembled tabular performance data exists for all building materials and is used to design structures for specific load conditions.
When I design a structure I don't have to test it to see if it will stand and carry the required loads because I base the design on established empirically derived engineering data.
LERA went through a similar process when the designed the towers. However they DID do some wind tunnel testing, which at the time was very new for buildings. It was done to determine if their unconventional approach to wind shear was sound. The wind tunnel test determined that the building moved too much in anticipated wind loads and so visco elastic damping was added to the the truss to facade connection attached to from each truss to every other facade column location.
Further, there were concrete slump tests done as per usual to determine that the concrete mix met designed specs. We don't have the construction logs however showing those test results.
Some of the steel was tested as well, random and destructive testing to determine if it was as per spec. Much of the discussion of how the design loads and materials and systems is in the NIST NCSTAR 1A report and makes interesting reading.
I've stated many times that the twin towers were a very unusual design in that they had their floors constructed in assemblies off site and were very long span with the columns to support one side of the floor slabs also used as the mechanism to resist wind sheer. And the facade was likewise custom fabricated off site.
Further the columns required for the bottom all were custom fabricated box sections of welded plates. There were no rolled section available to support the calculated axial loads. Their design was determined, not by testing them... as it is rather impossible to place a load of 32 million pounds on the sample to see if it would fail. This was the yield strength of core column 501 for example.
The formulas which describe Euler buckling were empirically determined centuries ago and probably a the basis for some experiments in schools. But in the real world Euler's formulas and Young's modulus are used all the time to mathematically test and design structures,
The towers were MOSTLY bolted together as well, but welds are tested as well...
What I have described is basic engineering and is exactly what is predicted for any structure which is over stressed, or over loaded. The key here is that the floor failure was a sub set of the entire structure... and the failure of one floor led to the failure of the identical floor below and then to the columns. Once the load imbalance (excessive) was introduced to the floors.. by redistributing load ONTO them which were formerly carried by the columns... the floors HAD to fail.
The reason why you don't see other high rises come down like this is because presenting such excessive over loading to the upper floors is almost a virtual impossibility under normal circumstances. The destructive load came from the top section of the tower itself as it was somehow destroyed (14 floors in tower 1) and no longer supported by the columns in increments at each floor level, but became an imposed live dynamic load on the upper most intact floor... a live load which was more than 20 times the floor's yield strength.
Go place a ton on every square foot of the top floor of Sears tower and watch all the floors below it collapse exactly as the twin towers.... or any high rise structure for that matter... steel or concrete frame. No heat required what so ever. If you drop the ton on it... its force is multiplied as a dynamic load.
Engineers analyze the performance of structures without physical testing. But they do test complex structures such as autos in destructive testing... or airplanes in wind tunnels.
Absolutely no real need to do physical tests to prove what has been proven about material performance. And scale models will not be reliable. You can't scale all factors in a test such as time or gravity. But if you want to see Euler buckling at work try to assemble a 1/100 scale model of column 501. it will stand 13.6 feet high made of 38 segments that are 4.3 inches tall by .52" x .22 in cross section at the base and H sections which are .14 x .14 made of material which is .01 thick. Place these one atop the other with a dab of glue and see how stable the 13.62' column is. It wont be and it wont be an accurate test of column 501 either. You can't scale model structural failures!
Every time you drive over a suspension bridge consider that it was designed on paper and probably with a slide rule and little or no testing other than noted above.. slump tests and random materials destructive testing. When the engineering is done properly and erected to spec the structure performs as designed under the load conditions EACH component was designed for. If you overload an elevator, you can fail it's cables or car structure etc. but the building it is contained in will NOT fail.
If you overload the top floor of any number of identical spec'd floors and it collapses... all the same floors below will then ALSO collapse when that floor's loads are presented to the ones below. Basic engineering... not a hypothesis or theory.
If you don't accept or understand these basic concepts, you have no business discussing the destruction of a building which came down... came apart and lost its structural integrity. People are only exposing their own ignorance of statics, and engineering by stating that it is impossible for a building such as the twin towers to fail as we observed.
I do not present a theory or an hypothesis but present the engineering principles in use every day to design structures.
The strength of materials has been tested and is presented in design load tables. Take out your ASTM steel design manual or even Wood Structural Design data for example and you will find the design loads for various types of steel or wood, sections, connections and so forth. The assembled tabular performance data exists for all building materials and is used to design structures for specific load conditions.
When I design a structure I don't have to test it to see if it will stand and carry the required loads because I base the design on established empirically derived engineering data.
LERA went through a similar process when the designed the towers. However they DID do some wind tunnel testing, which at the time was very new for buildings. It was done to determine if their unconventional approach to wind shear was sound. The wind tunnel test determined that the building moved too much in anticipated wind loads and so visco elastic damping was added to the the truss to facade connection attached to from each truss to every other facade column location.
Further, there were concrete slump tests done as per usual to determine that the concrete mix met designed specs. We don't have the construction logs however showing those test results.
Some of the steel was tested as well, random and destructive testing to determine if it was as per spec. Much of the discussion of how the design loads and materials and systems is in the NIST NCSTAR 1A report and makes interesting reading.
I've stated many times that the twin towers were a very unusual design in that they had their floors constructed in assemblies off site and were very long span with the columns to support one side of the floor slabs also used as the mechanism to resist wind sheer. And the facade was likewise custom fabricated off site.
Further the columns required for the bottom all were custom fabricated box sections of welded plates. There were no rolled section available to support the calculated axial loads. Their design was determined, not by testing them... as it is rather impossible to place a load of 32 million pounds on the sample to see if it would fail. This was the yield strength of core column 501 for example.
The formulas which describe Euler buckling were empirically determined centuries ago and probably a the basis for some experiments in schools. But in the real world Euler's formulas and Young's modulus are used all the time to mathematically test and design structures,
The towers were MOSTLY bolted together as well, but welds are tested as well...
What I have described is basic engineering and is exactly what is predicted for any structure which is over stressed, or over loaded. The key here is that the floor failure was a sub set of the entire structure... and the failure of one floor led to the failure of the identical floor below and then to the columns. Once the load imbalance (excessive) was introduced to the floors.. by redistributing load ONTO them which were formerly carried by the columns... the floors HAD to fail.
The reason why you don't see other high rises come down like this is because presenting such excessive over loading to the upper floors is almost a virtual impossibility under normal circumstances. The destructive load came from the top section of the tower itself as it was somehow destroyed (14 floors in tower 1) and no longer supported by the columns in increments at each floor level, but became an imposed live dynamic load on the upper most intact floor... a live load which was more than 20 times the floor's yield strength.
Go place a ton on every square foot of the top floor of Sears tower and watch all the floors below it collapse exactly as the twin towers.... or any high rise structure for that matter... steel or concrete frame. No heat required what so ever. If you drop the ton on it... its force is multiplied as a dynamic load.
Engineers analyze the performance of structures without physical testing. But they do test complex structures such as autos in destructive testing... or airplanes in wind tunnels.
Absolutely no real need to do physical tests to prove what has been proven about material performance. And scale models will not be reliable. You can't scale all factors in a test such as time or gravity. But if you want to see Euler buckling at work try to assemble a 1/100 scale model of column 501. it will stand 13.6 feet high made of 38 segments that are 4.3 inches tall by .52" x .22 in cross section at the base and H sections which are .14 x .14 made of material which is .01 thick. Place these one atop the other with a dab of glue and see how stable the 13.62' column is. It wont be and it wont be an accurate test of column 501 either. You can't scale model structural failures!
Every time you drive over a suspension bridge consider that it was designed on paper and probably with a slide rule and little or no testing other than noted above.. slump tests and random materials destructive testing. When the engineering is done properly and erected to spec the structure performs as designed under the load conditions EACH component was designed for. If you overload an elevator, you can fail it's cables or car structure etc. but the building it is contained in will NOT fail.
If you overload the top floor of any number of identical spec'd floors and it collapses... all the same floors below will then ALSO collapse when that floor's loads are presented to the ones below. Basic engineering... not a hypothesis or theory.
If you don't accept or understand these basic concepts, you have no business discussing the destruction of a building which came down... came apart and lost its structural integrity. People are only exposing their own ignorance of statics, and engineering by stating that it is impossible for a building such as the twin towers to fail as we observed.